This invention relates to a superluminescent diode, more particularly to a superluminescent diode that can be easily fabricated and efficiently coupled into a single-mode optical fiber.
A superluminescent diode is a light source with properties intermediate between those of a light-emitting diode (LED) and laser diode (LD). Superluminescent diodes can be tailored to the requirements of a wide variety of optical systems. They have been used in fiber-optic transmission systems, for example, and for optical pumping of lasers and fiber-optic gyroscope.
Prior-art superluminescent diodes have had a ridge structure, the ridge acting as a waveguide for lateral confinement of light emitted by an active layer underlying the ridge. The output facet of the diode is provided with an antireflection coating of, for example Si.sub.3 N.sub.4. The opposite facet of the diode is provided with a high-reflectivity mirror coating comprising, for example, a gold layer and an Si.sub.3 N.sub.4 layer. Spontaneous emission occurring in the active layer is amplified by further stimulated emission on the extended light path within the waveguide, possibly including one mirror reflection. The antireflection coating, however, prevents optical feedback, so the diode does not operate as a laser. Its output is therefore free of the modal, partition, and feedback noise present in laser diode output.
Operating at an ambient temperature of 20.degree. C. with a drive current of 250 mA, a prior-art superluminescent diode of this structure can provide an output power of, for example, 1.9 mW. The output can be coupled into a 50-micron core diameter multi-mode fiber with an efficiency of, for example, 30%.
The coupling efficiency into a single-mode fiber, however, is lower, because the output region at the end of the ridge is too wide in relation to the single-mode fiber core diameter. This makes the prior-art superluminescent diode unsuitable for certain applications, such as in fiber-optic interference sensors. In addition, reliable fabrication of an antireflection coating with the necessary low reflectivity is difficult. Lasing is therefore apt to occur in prior-art superluminescent diodes, due to unavoidable manufacturing variability.